Various types of apparatuses for transferring a workpiece between press machines have conventionally been proposed.
For example, as a conventional workpiece transfer apparatus including a crossbar unit extending in a direction approximately orthogonal to a workpiece transfer direction, a workpiece transfer apparatus including a crossbar, a workpiece retaining device, a threaded shaft, a rotary driving device, and a guiding device is known. In the conventional workpiece transfer apparatus, the crossbar is reciprocated in the workpiece transfer direction. The workpiece retaining device is provided to the crossbar so as to retain and release the workpiece. The threaded shaft is provided turnably along a longitudinal direction of the crossbar and is threadably engaged with the workpiece retaining device. The rotary driving device rotationally drives the threaded shaft. The guiding device guides and supports the workpiece retaining device by the rotary drive of the threaded shaft so that the workpiece retaining device is movable along the longitudinal direction of the crossbar.
Japanese Patent Application Laid-Open No. 2003-290850 describes the following workpiece transfer apparatus. As illustrated in the accompanying drawings of Japanese Patent Application Laid-Open No. 2003-290850, the workpiece transfer apparatus includes one threaded shaft (20) provided to at least one of crossbars (7) so as to be turnable along a longitudinal direction of the crossbar (7), the threaded shaft including external screws formed at a plurality of positions on an outer circumferential portion thereof, and rotary driving means (25) for rotating the threaded shaft (20). At least each one of a plurality of workpiece retaining means (9) is threadably engaged with each of the plurality of external screws of the threaded shaft (20) so as to be movable along the longitudinal direction of the crossbar (7). A workpiece (11) is transferred for a next process by the retention of the workpiece by the workpiece retaining means (9), the vertical movement of the crossbar (7), and the reciprocation of the crossbar (7) in a workpiece transfer direction.
Japanese Patent Application Laid-Open No. 2003-290851 describes another workpiece transfer apparatus. As illustrated in the accompanying drawings of Japanese Patent Application Laid-Open No. 2003-290851, the workpiece transfer apparatus for a press machine includes a crossbar (7) provided in a direction approximately orthogonal to a workpiece transfer direction, the crossbar being provided with workpiece retaining means (9) capable of retaining a workpiece, a linear motor (17) for allowing the workpiece retaining means (9) provided to at lease one crossbar (7) to be movable along a longitudinal direction of the crossbar (7) and for driving the movement of the workpiece retaining means (9), and a controller (14) for controlling the linear motor (17) in synchronization with an operation of the press machine and/or solely.
Japanese Patent Application Laid-Open No. Hei 11-57899 describes still another workpiece transfer apparatus. As illustrated in the accompanying drawings of Japanese Patent Application Laid-Open No. Hei 11-57899, guide beams (20), which are obtained by detachably connecting guide beam members (19), each being modularized per workpiece transfer pitch unit, in a longitudinal direction, are fixed in parallel on the right and left sides opposed to each other through a press line (L) inside a press housing (4). A guide rail (21) extending along the longitudinal direction is provided on one end surface of each of the guide beam, members (19) of the guide beams (20). A servo motor 24 for lifting, for allowing a crossbar (13) to perform an upward/downward movement operation is assembled to a crossbar carrier (22) movably engaged with and supported by the guide rail (21). A servo motor (27) for feeding, which allows the crossbar (13) to perform a transfer operation through an intermediation of the crossbar carrier (22) by a rack and pinion mechanism is provided.
The workpiece transfer apparatuses, each including the crossbar unit extending in the direction approximately orthogonal to the workpiece transfer direction, as described in Japanese Patent Applications cited above, can automatically change the location of the workpiece retaining means which retains the workpiece by vacuum adsorption or magnetic attraction according to the size or shape of the workpiece to be processed, and hence a preparatory operation can be performed easy, rapid, and precise. In view of the advantage mentioned above, the workpiece transfer apparatuses described above are beneficial.
On the other hand, however, there is a demand to increase a workpiece transfer speed, and eventually to increase a press working speed (to improve production efficiency). The “threaded shaft” and the “rack” extending along the crossbar are relatively heavy in weight as found in the conventional workpiece transfer apparatuses. Therefore, even if the workpiece transfer speed (speed of reciprocation of the crossbar) is to be increased, the workpiece transfer speed cannot be increased above a predetermined level under the actual conditions due to, for example, large vibrations caused by a large inertia force.
For a reduction in weight, for example, it is conceivable to reduce a length of the “threaded shaft” or the “rack”. With the reduction in length, however, the range where the workpiece retaining means can move is disadvantageously limited under the actual conditions.
Further, in the conventional configuration using the “threaded shaft” or the “rack”, it is necessary to supply oil such as grease between members which are threadably engaged with each other or are meshed with each other. Therefore, there is a fear in that the supplied grease may adhere to the workpiece to adversely affect a subsequent process(es) and quality of products.